This invention relates to ultrasonic diagnostic imaging systems and, in particular, to a method and apparatus for obtaining three-dimensional ultrasonic Doppler images of moving sound reflectors in blood and tissues.
A variety of ultrasound imaging modalities have been developed to suit a variety of specific applications. For example, Doppler imaging has been developed to allow the imaging of moving ultrasound reflectors. Doppler ultrasound imaging systems detect a Doppler shift in the frequency of a transmitted signal reflected from ultrasound reflectors, and display returns only from such reflectors. The magnitude of the Doppler shift corresponds to the velocity of the ultrasound reflectors, and the polarity of the Doppler shift corresponds to the direction of movement. Conventional Doppler images are thus able to provide an indication of both blood flow velocity and blood flow direction, thereby allowing arterial blood flow to be differentiated from venous blood flow. Doppler imaging can also be used to visualize the movement of tissues, such as heart wall movement.
Although Doppler imaging provides a great deal of clinically useful information, Doppler imaging is not without its problems and limitations. The magnitude of the Doppler shift corresponds to the projection of the velocity of the blood flow on the ultrasound beam. The Doppler shift from blood flowing at an angle to the axis of the ultrasound beam corresponds to the product of the blood flow velocity and the cosine of the angle between the direction blood flow direction and the axis of the beam. Therefore, the velocity of blood flow can be accurately determined and portrayed in an ultrasound Doppler image only if the angle between the blood flow and the axis of the ultrasound beam is known. Yet it can be difficult to make this determination.
Even if the angle between the axis of the ultrasound beam and an artery or vein is known, it can still be difficult or impossible to accurately determine the velocity of blood flowing through the blood vessel because the flow of blood through a vessel is not always aligned with the axis of the vessel. Blood can flow through a blood vessel in a helical manner. Furthermore, the flow of blood in a blood vessel becomes even more irregular in the presence of bends, bifurcations or obstructions in the vessel. Thus, a single cosine correction angle cannot be used to accurately correct signals indicative of the velocity of moving reflectors in an artery or vein.
In conventional Doppler imaging systems, a two-dimensional Doppler image is obtained by using an ultrasound transducer having a linear, one-dimensional array of transducer elements. Signals applied to or received from the array are combined to form a beam that is steered by phase-shifting the signals to sample locations in a two-dimensional plane. If each sample location in the two-dimensional plane is interrogated from two different apertures, i.e., by two different beams emanating from different locations, the absolute mean velocity of flow at that sample location can be determined in two dimensions. However, such systems are incapable of accurately portraying the true flow velocity because the true velocity may have a component that is perpendicular to the two-dimensional plane.
One approach to determining blood flow in three dimensions is disclosed in U.S. Pat. No. 5,522,393 to Philips et al., which discloses a system using a transducer having a non-planar phased array that interrogates each sample volume using three independently steered beams. Although the two-dimensional phased arrays taught by the Philips et al. patent are capable of accurately determining the velocity of blood flow in three dimensions, the structure of the transducers disclosed in the Philips et al. patent make them difficult to use. In particular, because the faces of the arrays are curved, it can be difficult to maintain good acoustic contact with the surface of tissues to be imaged unless the curvature of the surface is substantially the same as the curvature of the face of the array. However, the curvature of the array face will not generally match the curvature of the surface of tissues to be imaged. The approach described in the Philips et al. patent thus has a limited range of applications. Furthermore, the large number of elements in the array each located in a different three-dimensional position produce respective signals that can be combined only with a great deal of computational complexity.
There is therefore a need for a system and method for providing a three-dimensional Doppler image using an ultrasound transducer that can be used with relative ease and that produces signals that can be combined to create the image with relatively little computational complexity. Furthermore such a system should be capable of imaging both blood flow and tissue motion, in order to determine the true velocity of both heart and vessel wall motion. Delineation of the direction of motion will both make the diagnosis easier and allow better understanding of the source of the motion abnormality.
An ultrasonic imaging system for generating a three-dimensional Doppler image includes a scanhead and an imaging unit. The scanhead includes a transmit aperture, and at least three receive apertures arranged in a common plane. The imaging unit includes a beamformer coupled to the receive apertures. The beamformer combines signals from several transducer elements in each of the receive apertures to generate signals indicative of ultrasound Doppler returns from a selected volume adjacent the receive aperture. Respective Doppler processors for the receive apertures generate respective magnitude signals indicative of the Doppler flow magnitude of moving ultrasound reflectors in the selected volume and a direction signal indicative of the direction of the moving ultrasound reflectors in the selected volume. A velocity estimator is coupled to receive the magnitude and direction signals from each of the Doppler processors. The velocity estimator generates a magnitude signal indicative of the magnitude of a three-dimensional flow vector corresponding to the magnitude signals from the Doppler processors and a flow angle signal indicative of the direction of the three-dimensional flow vector corresponding to the direction signals from the Doppler processors. The imaging system also includes a display processor coupled to receive the magnitude signal and the angle signal from the velocity estimator. The display processor converts the magnitude and angle signals to display signals having a predetermined display format.
The transmit aperture is preferably positioned symmetrically between the receive apertures and in the common plane of the receive apertures. In one aspect of the invention, the scanhead may include a first pair of receive apertures positioned in the common plane along a first axis, and a second pair of receive apertures positioned in the common plane along a second axis that is perpendicular to and intersects the first axis. In this configuration, the transmit aperture may be positioned in the common plane at the intersection of the first and second axes. In another aspect of the invention, the scanhead may include a plurality of receive apertures equally spaced from a center point of the scanhead and circumferentially spaced from each other. The receive apertures may have a hexagonal shape, and the transmit aperture may be centered at the center point between the receive apertures.